1. Field of the Invention
This invention relates to semiconductor fabrication and, more particularly, to an integrated circuit having a dissimilarly placed graded junction produced by the selective formation or removal of a metal spacer.
2. Description of Relevant Art
Fabrication of a metal-oxide semiconductor ("MOS") transistor is well known. The manufacture of an MOS transistor begins by defining active areas where the transistor will be formed. The active areas are isolated from other areas on the semiconductor substrate by various isolation structures formed upon and within the substrate. Isolation structures come in many forms. For example, the isolation structures can be formed by etching trenches into the substrate and then filling the trenches with a dielectric fill material. Isolation structures may also be formed by locally oxidizing the silicon substrate using the well recognized LOCOS technique.
Once the isolation structures are defined between transistor active areas, a gate dielectric is formed. Typically, the gate dielectric is formed by thermal oxidation of the silicon substrate. Thermal oxidation is achieved by subjecting the substrate to an oxygen-bearing, heated ambient in, for example, an oxidation furnace or a rapid thermal anneal ("RTA") chamber. A gate conductor material is then deposited across the entire dielectric-covered substrate. The gate conductor material is preferably polycrystalline silicon, or polysilicon. The polysilicon layer is then patterned using a photolithography mask. The mask allows select removal of a light-sensitive material deposited entirely across polysilicon. The material which is exposed can, according to one embodiment, be polymerized, and that which is not exposed removed. Selective polymerization is often referred to as the "develop" stage of lithography. The regions which are non-polymerized are removed using the etch stage of lithography.
An n-channel transistor, or NMOS transistor, must in most instances be fabricated different from a p-channel transistor, or PMOS transistor. NMOS transistors employ n-type dopants on opposite sides of the NMOS gate conductor, whereas PMOS transistors employ p-type dopants on opposite sides of the PMOS transistor gate conductor. The regions of the substrate which receive dopants on opposite sides of the gate conductor are generally referred to as junction regions, and the distance between junction regions is typically referred to as the physical channel length. After implantation and subsequent diffusion of the junction regions, the distance between the junction regions become less than the physical channel length and is often referred to as the effective channel length ("Leff"). In high density designs, not only does the physical channel length become small so too must the Leff. As Leff decreases below approximately 1.0 .mu.m, for example, a problem known as short channel effects ("SCE") becomes predominant.
A problem related to SCE, and the subthreshold currents associated therewith, but altogether different is the problem of hot-carrier effects ("HCE"). HCE is a phenomenon by which hot-carriers ("holes and electrons") arrive at or near an electric field gradient. The electric field gradient, often referred to as the maximum electric field ("Em"), occurs near the drain during saturated operation. More specifically, the electric field is predominant at the lateral junction of the drain adjacent to the channel. The electric field at the drain causes primarily electrons in the channel to gain kinetic energy and become "hot". These hot electrons traveling to the drain lose their energy by a process called impact ionization. Impact ionization serves to generate electron-hole pairs, wherein the pairs migrate to and become injected within the gate dielectric near the drain junction. Traps within the gate dielectric generally become electron traps, even if they are partially filled with holes. As a result, there is a net negative charge density in the gate dielectric. The trapped charge accumulates with time, resulting in a positive threshold shift in the NMOS transistor, or a negative threshold shift in a PMOS transistor. It is known that since hot electrons are more mobile than hot holes, HCE causes a greater threshold skew in NMOS transistors than PMOS transistors. Nonetheless, a PMOS transistor will undergo negative threshold skew if its Leff is less than, e.g., 0.8 .mu.m.
Unless modifications are made to the fabrication sequence, problems resulting from HCE will remain. To minimize these problems, a mechanism must be derived that disperses and thereby reduces Em. That mechanism is often referred to as the double-diffused drain ("DDD") and lightly doped drain ("LDD") techniques. The purpose behind using DDDs and LDDs structures is to absorb some of the potential into the drain and away from the drain/channel interface. The popularity of DDD structures has given way to LDD structures since DDD may cause unacceptably deep junctions and deleterious junction capacitance.
A conventional LDD structure is one whereby a light concentration of dopant is self-aligned to the edge of the gate conductor. The light-dopant concentration is then followed by a heavier-dopant concentration which is self-aligned to a spacer formed on the sidewalls of the gate conductor. The purpose of the first implant dose is to produce a lightly doped section of both the source and drain junction areas at the gate edge near the channel. The second implant dose is spaced from the channel a distance dictated by the thickness of the sidewall spacer. Resulting from the first and second implants, a dopant gradient occurs across the junction from the source/drain area of the junction to the LDD area adjacent the channel.
The dopant gradient across the junction, henceforth referred to as a graded junction, is necessary for several reasons. First, the lightly doped region (LDD area) is used to assume a substantial portion of the entire voltage drop associated with Em. It has been reported that the LDD area may in some instances reduce Em at the drain juncture by approximately 30-40%. Secondly, the heavier dosage within the source/drain area forms a low resistivity region suitable for enhanced contact conductivity. Further, the source/drain dose is implanted at a higher energy necessary to produce deeper source/drain junctions and thereby provide better protection against junction spiking.
The benefits of using an LDD area in conjunction with a source/drain area is generally well documented. However, the benefits differ depending upon whether an NMOS device or a PMOS device is produced. For example, an NMOS device requires an LDD area more so than a PMOS device. However, an unduly large LDD area would dampen NMOS performance by increasing the source-drain resistance. On the other hand, the dopants used to form a PMOS device (i.e., boron) are more mobile than the dopants used to form an NMOS device. As such, the p-type dopants regularly segregate and migrate from their original implant position toward and into the channel area. This lessens Leff and produces deleterious SCE problems.
It would therefore be desirable to employ a CMOS fabrication process which can produce NMOS junctions dissimilar from PMOS junctions. If the NMOS junction is graded such that the LDD area is relatively small, then performance can be enhanced. Similarly, if the PMOS junction can be graded such that the highly mobile source/drain implants are drawn farther from the channel then SCE can be more carefully controlled.